In the light of improvement of connection workability of an optical connector ferrule, it has been recently provided a so-called MPO connector (an optical connector having a structure in which a multi-fiber optical connector ferrule stipulated in the JIS-05982 standard is housed within a plastic housing).
<Conventional Structure>
Conventionally, as shown in FIGS. 9A and 9B, an MPO type optical connector 1 has a structure in which a ferrule 3 attached to the leading end of an optical fiber cable (an optical cord) 3b is housed in the head (the front end) of a sleeve-shaped housing 2. Further, a coil spring 5 elastically urging the ferrule 3 toward the front side of the optical connector 1 is provided within the housing 2.
Further, the MPO type optical connector 1 is provided with a housing 2 which is formed into a tubular shape having a substantially rectangular cross section, and is formed with a concave groove 2c for housing a spring on an outer surface of both sides at the rear of the optical connector 1, a tubular coupling 7 which is slidably provided within a movable range secured in an axial direction with respect to the housing 2, and is elastically urged toward the front side of the optical connector 1 by a spring 8 housed in the concave groove 2c of the housing 2, a spring push 6 which is attached to the rear end opposing to the front end in which the ferrule 3 of the housing 2 is provided, and a protective boot 9 which externally supprounds the rear end of the spring push 6. On an outer surface of the head (the front end) of the housing 2, a key projection K1 is formed, which is inserted along a key groove K2 formed on an inner surface of the coupling 7 (see FIGS. 11, 12A, 12B, 12C, 12D and 12E).
As shown in FIGS. 10A to 10E and 11, the housing 2 is formed into a tubular shape having a substantially rectangular cross section, and a locking hole 2a into which the spring push 6 is locked is oppositely formed on both sides at the rear of the housing 2. Reference numeral 2d in the drawings denotes an engagement groove, and the engagement groove 2d engages with a locking projection formed in an optical adapter that is a connection counterpart thereof.
In the meantime, the spring push 6 attached to the housing 2 by inserting thereinto is formed into a substantially U-shape in a plan view and can be inwardly bent each other, and a locking projection 6a to be locked into the locking hole 2a of the housing 2 is formed on an outer surface of both sides at the head of the spring push 6. Further, a fitting portion 6c having a stepped and cylindrical shape is provided at the rear of the spring push 6, and the leading end of the protective boot 9 is fitted to the fitting portion 6c via a tubular caulking ring 6d whose intermediate portion is stepwisely constricted. The protective boot 9 fitted to the fitting portion 6c abuts at its leading end against a stepped surface 6e at the base of the fitting portion 6c, thereby restricting a displacement of the protective boot 9 to a front direction of the spring push 6. Here, in a state where the locking projection 6a at the head of the spring push 6 is locked into the locking hole 2a at the front of the housing 2, the fitting portion 6c of the spring push 6 is positioned within the housing 2 closer to the front end side of the housing 2 than the rear end side thereof.
As mentioned above, the spring push 6 to be locked into the housing 2 retains a coil spring 5 lest the coil spring 5 comes off the housing 2. Here, the coil spring 5 presses the ferrule 3 in a fitting direction via a pin base 4.
The insertion of the spring push 6 into the optical adapter (not shown) of the connector main body 1 is achieved by pressing a portion positioned at the rear side farther from the coupling 7 of the connector main body 1, for example, the protective boot 9 toward the optical adapter while gripping the protective boot 9 with a hand and fingers of a worker.
Conventionally, the extraction of the spring push 6 from the optical adapter of the connector main body 1 is achieved by pulling in the coupling 7 to the protective boot 9 side against an elastic urging force of the spring 8.
Recently, the MPO type optical connector 1 has been under the pressure of necessity of downsizing itself with the advance of high densification and space saving in a server rack of a data center. The optical connector 1 downsized as above is housed in an extremely bent state when housed in the rack, and it is often the case that a load (a bending stress) is applied to the optical connector 1, particularly to the optical fiber cable 3b. 
For example, the optical fiber cable 3b wired in a rear portion of the rack in which an optical communication device is accommodated is bent in the vicinity of a backboard of the optical communication device, and is connected to the optical fiber cable 3b at the optical communication device side. In a case where the optical fiber cable 3b is bent as above, it is generally said to necessary for maintaining a radius of curvature more than 30 mm, taking account of a light loss caused by the bending thereof.
For that reason, it has been conventionally taking measures such that the optical fiber cable 3b is inserted into the protective boot 9 having flexibility, and the aforesaid radius of curvature of the optical fiber cable 3b is maintained by a bending range allowed for the protective boot 9, at a portion at the rear of the housing 2 of the optical connector 1.
<Structure of Patent Literature 1>
In some cases, when the protective boot 9 is excessively bent, it is infrequently happens that the protective boot 9 comes off the optical connector 1. To avoid such a situation, it has been conventionally proposed an optical connector which can secure a bending radius of the optical fiber cable 3b with a reduced space taking account of a bending loss and breakage of the optical fiber cable 3b as disclosed, for example, in the Patent Literature 1.
Namely, in the aforesaid Patent Literature 1, the optical connector 1 has a receiving portion corresponding to the fitting portion 6c in a stopper 6 (a spring push), and the locking projection 6a in an extending portion on both sides of the stopper 6. Further, the leading end of the receiving portion forms the rear end of the stopper 6 (the spring push). Furthermore, an arm extends in a backward direction thereof through both sides of the receiving portion.
Between two arms, there is provided a hooking portion which prevents the protective boot 9 from being displaced toward the rear end side of the housing 2. The hooking portion is a substantially rectangular plate which is positioned on a surface in parallel with the rear end, and has at its center a substantially rectangular hole.
Further, when the protective boot 9 is attached from the rear side of the stopper 6 (the spring push), the dimensions of the hole in a height direction and a width direction are set to be greater than the dimensions of the receiving portion in a height direction and a width direction so that a leading end of the protective boot 9 can be fitted to the receiving portion. Furthermore, a stop surface corresponding to the fitting portion 6c for the protective boot 9 is provided opposing to the hooking portion. Here, a surface of the hooking portion opposing to the stop surface forms a hooking surface.
The protective boot 9 has convex portions 29c, each protruding substantially in an arc shape, on two peripheral surfaces of the leading end thereof in a width direction. A height and a width of the leading end of the protective boot 9 are formed to be somewhat smaller than a height and a width of the hole of the stopper 6 (the spring push). Further, the height of the leading end of the protective boot 9 including the convex portions is set to be higher than the height of the hole. A thickness (as seen from a length direction of the protective boot 9) of the convex portions is set to be somewhat thinner (smaller) than a distance between the stop surface of the protective boot 9 and the hooking surface. An inner periphery of the leading end of the protective boot 9 is formed to be somewhat smaller than an outer periphery of the receiving portion of the stopper 6 (the spring push).
The leading end of the protective boot 9 thus configured as above is fitted to the receiving portion of the stopper 6 (the spring push) from the arm side of the stopper 6 (the spring push) through the hole. The height of the leading end of the protective boot 9 including the convex portions is higher than the height of the hole of the stopper 6 (the spring push). However, the leading end of the protective boot 9 can pass through the hole because the protective boot 9 has flexibility, that is, the protective boot 9 is elastically deformable.
The leading end of the protective boot 9 fitted to the receiving portion through the hole of the stopper 6 (the spring push) does not easily come off the receiving portion because an inner periphery thereof is formed to be somewhat smaller than an outer periphery of the receiving portion. Further, the convex portions provided at the leading end of the protective boot 9 are positioned between the stop surface of the stopper 6 (the spring push) and the hooking surface, and hook the protective boot 9 to the stopper 6 (the spring push).
Therefore, the protective boot 9 is extremely hard to come off the optical connector 1 because the leading end of the protective boot 9 is fitted to the receiving portion of the spring push 6, and the convex portions of the protective boot 9 are hooked to the spring push 6 in the optical connector 1.